The main material of a tube for automobile piping is shifting from a metal to a lightweight resin, with an excellent rust-preventing property, so as to overcome an old problem of rusting due to anti-freeze agents on roads or to meet the recent requirement for energy saving which is a pressing issue in view of the prevention of global warming. The resin usually used for the piping tube includes a polyamide-based resin, a saturated polyester-based resin, a polyolefin-based resin, a thermoplastic polyurethane-based resin and the like. However, a single-layer tube using such a resin is insufficient in heat resistance, chemical resistance and the like and, therefore, the applicable range thereof is limited.
In the tube for automobile piping, from the standpoint of reducing gasoline consumption and attaining higher performance, an oxygen-containing gasoline having blended therein alcohols, having low boiling points, such as methanol and ethanol, or ethers such as methyl-tert-butyl ether (MTBE), is transferred. Furthermore, in view of environmental antipollution, strict regulations regarding exhaust gas have been recently implemented, including preventing volatile hydrocarbons or the like from leaking out into air by diffusion through a piping tube walls. The regulations will become more and more strict in the future and it is desired to maximally prevent the fuel from permeating and evaporating through the piping tube walls. Such strict regulations cannot be satisfied by a conventionally-employed single-layer tube using a polyamide-based resin, and particularly polyamide 11 or polyamide 12 excellent in the strength, toughness, chemical resistance and flexibility, because the capability of preventing permeation of the above-described fuel is insufficient. Thus, an improvement is required in the prevention of, particularly, alcohol gasoline permeation.
As for the method to solve this problem, there has been proposed a multilayer tube having disposed therein a resin having good alcohol gasoline permeation-preventing property, such as an ethylene-vinyl acetate copolymer saponification product (EVOH), polymetaxylylene adipamide (polyamide MXD6), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), ethylene/tetrafluoroethylene copolymer (ETFE), ethylene/chlorotrifluoroethylene copolymer (ECTFE), tetrafluoroethylene/hexafluoropropylene copolymer (TFE/HFP, FEP), tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (TFE/PAVE, PFA) and tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride copolymer (TFE/HFP/VDF, THV) (see, for example, Kohyo (National Publication of Translated Version) No. 7-507739).
Furthermore, a fuel transfer tube where a fluorine-based resin is disposed as an inner layer and a layer comprising an ethylene/vinyl acetate copolymer saponification product is disposed outside the inner layer has been proposed (see, Kokai (Japanese Unexamined Patent Publication) No. 5-247478). However, the ethylene/vinyl acetate copolymer saponification product (EVOH) used in this technique is lacking in the thermal stability and when the die temperature, in the merging section, is high during the coextrusion molding with a fluorine-based resin, the polymer may undergo thermal deterioration. In order to prevent this problem, the melting point of the fluorine-based resin may be reduced by controlling the compositional ratio of monomers thereof and rendered close to the processing temperature of the ethylene/vinyl acetate copolymer saponification product, but the alcohol gasoline permeation-preventing property inevitably decreases.
On the other hand, a fuel transfer tube where a fluorine-based resin is disposed as an inner layer and a layer comprising a partial aromatic polyamide resin is disposed outside the inner layer has been proposed (see, Kokai No. 5-245988). However, the polymetaxylylene adipamide (polyamide MXD6) disclosed as the partial aromatic polyamide in Examples of this technique is insufficient in the adhesive strength to polyamide 11 and/or polyamide 12 conventionally employed as a material constituting the tube for automobile piping, and an adhesive layer such as modified polyolefin needs to be provided between those layers. When a modified polyolefin is used as the adhesive layer, this is disadvantageous in that the thermal aging resistance is poor and the tube cannot be used in severe conditions, and fails in maximally bringing out the characteristic properties of the partial aromatic polyamide resin or fluorine-based resin constituting the fuel transfer tube. Also, the increase in the number of layers may incur problems in view of cost and process control.
Furthermore, a fuel transfer tube comprising an innermost layer formed of a fluorine-based resin, an intermediate layer formed of a polyalkylene naphthalate resin and an outer layer formed of a thermoplastic resin or a thermoplastic elastomer, wherein an adhesive layer is disposed between the innermost layer and the intermediate layer, has been proposed (see, Kokai No. 7-96564). The fuel transfer tube of this technique is very excellent in the alcohol gasoline permeation-preventing property. In this fuel transfer tube, an adhesive resin composition comprising at least one member selected from the group consisting of fluorine-based resins and fluorine-based rubbers, at least one member selected from the group consisting of crystalline polyester-based resins and polyester-based elastomers, and a compatibilizing agent is proposed as the adhesive resin interposed between those layers. However, its interlayer adhesion is affected by the morphology of the mixture used as the adhesive resin and this gives rise to a problem that the interlayer adhesion greatly varies or decreases depending on the extrusion conditions, use environment conditions or the like. In particular, the durability of the interlayer adhesive strength during the immersion in fuel (hereinafter referred to as “fuel resistance of interlayer adhesive strength”) is poor. Also, in a fuel transfer tube comprising an innermost layer formed of a polyamide-based resin, an intermediate layer formed of a polyalkylene naphthalate resin and an outer layer formed of a thermoplastic resin or a thermoplastic elastomer, wherein an adhesive layer is disposed between the innermost layer and the intermediate layer, it is proposed to dispose, as the adhesive resin interposed between those layers, an adhesive resin mixture comprising at least one member selected from the group consisting of polyamide-based resins and polyamide-based elastomers, at least one member selected from crystalline polyester-based resins and polyester-based elastomers, and a compatibilizing agent. However, for the same reason as given above, there is a problem that the interlayer adhesion greatly varies or decreases depending on the extrusion conditions, use environment conditions or the like. Furthermore, in the case where the innermost layer is a polyamide-based resin, the resistance to sour gasoline, produced by oxidation of gasoline, (chemical resistance) is sometimes poor.
It would therefore be helpful to provide a multilayer tube excellent in an alcohol gasoline permeation-preventing property, interlayer adhesion, low-temperature impact resistance, heat resistance and/or chemical resistance.